Alternating current operated direct current amplifier circuits



1942- v F. 'H. SHEPARD, JR 73,150

ALTERNATING CURRENT OPERATED DIRECT CURRENT AMPLIFIER CIRCUITS OriginalFiled June 29, 1938 2 Sheets-Sheet 1 a mourn/r ac/A/Pur I FRANCIS H.5HRDA '1 7K5? Amman.

represent the line conductors of the A. C.

Patented Feb. i7, 1942 ALTERNATING CURRENT OPERATED DI- RECT CURRENTAMPLHIER CIRCUITS Francis H. Shepard, Jn, Merchantville, N. 3.

Original application June 29, 1938, Serial No.

216,448. Divided and this application Decemher 7, 1940, Serial No.369,060

7 Claims. (on. 179471) This application is a division of my U. S.application Serial No. 216,348, filed June 29,1938, Patent No.2,224,199, dated December 10, 1940.

The invention relates generally to D. C. amplifier circuits in which theenergy for the operation of the amplifier is derived from a source ofalternating current of the usual low frequency, and more particularly tosuch amplifiers of the balanced type, and is an improvement over myinvention described and claimed in co-pending application Serial No.727,968, filed May 28, 1934, and issued as Patent No. 2,137,419 onNovember 22, 1938. In that patent I have disclosed various types of A.C. operated D. C. amplifiers some of which are adjustable to providecompensation for all supply voltage variations. In order to avoid theinconvenience of a separate and rather critical compensation adjustment,such as is necessary in the circuits of the above patent, I haveconceived and tested several types of self-biased, balanced, A. C.operated D. C. amplifiers that tend to be inherently compensated for allsupply voltage variations including plate, bias and heater or filamentvoltage variations. The amplifiers according to my present invention maybe degenerative or regenerative, and any number of the circuits may becascaded without the necessity of cascading the power supply voltages.

My invention will be better understood from the following descriptionwhen considered in connection with the accompanying drawings and itsscope will bepointed out in the appended claims.

In the drawings Fig. 1 shows a single'stage A. C. operated D. C.amplifier utilizing a pair of tubes in balanced relation. Figs. 2 to8illustrate various modifications of the amplifier shown in Fig. 1. Fig.9 shows a pair of coupled balanced amplifiers. Fig. 10 shows amodification of one of the balanced stages of Fig. 9, and Fig. 11 showsa balanced amplifier according to my invention utilizing a single,multi-grid tube.

Referring now specifically to Fig. 1, Li, L2 power supply, between whichare connected in parallel relation a pair of tubes T, T of the triodetype,

although it will be understood that multi-grid, multi-purpose, or othersuitable tubes may be used in place thereof. The cathode K of each tubeis directly connected for example to one side L2 of the A. C. powersupply and the anodes are each connected to the other side Ll of thepower supply through resistor-capacity networks put or signal potentialto be measured is applied to the terminals 3, 4 which'are connectedrespectively to anode A of tube T through the resistor 5 and to thecontrol grid G. A condenser 6 connected between the cathode side of theline L2 and the side of the resistance Swhich is connected to inputterminal 3 forms with said resistance 5 a filter network which removesthe A. C. component from the plate circuit and supplies biasingpotential from the plate A to the control grid G of tube T and alsoopposes to a large extent the effect on the grid of the signal. voltageinput. A .similar resistance-capacity network 5', 6 is connected asshown to provide bias for the control grid of tube '1". The outputterminals 1 and 8 are connected to the respective low potential ends ofthe load resistors, l, l of tubes T and T. In the circuit just describedthe output of one tube is balanced against the output of the second,these tubes having preferably substantially similar characteristics sothat changes due to variations in the supply voltage will tend to becancelled in the common out- In considering the operation of this singlestage amplifier it should be understood that both sides of an A. C.power supply line are at the l-2, l'-2' and the potentiometer P. Theinsame D. C. potential, that is, a D. C. voltmeter placed between theopposite sides of the A. C. line will read zero. The current due to theelectron fiow from cathode K to anode A in tube T is in such directionthat the IR drop in the plate load resistor i causes the plate to be ata negative potential with respect to the voltage of supply line Ll. Theplate A of the tube T will go positive with respect to the cathode eachtime the upper side L! of the A.,C. line swings positive. During thistime electron current flows from the cathode to the plate and, asexplained above, the plate drops to a potential negative with respect tothe upper side Llof the A. C. line. The condenser 2 in shunt to theplate load resistor l is of such value as to hold its charge withoutappreciable lossfrom one cycle to the next of the power supply. Sincethe plate is negative with respect to theline Ll, it will also benegative with respect to the opposite side of the line L2 andhence aftertheremoval of the A. C. component of the potential by means of thefilter network comprising resistor 5 and condenser 6, this potential canbe applied in series with the signal input or else used as bias for thegrid of tube T. It will be seen that any change in input signal voltageresults in a change of grid potential which in turn changes the plate 8S8, vacuum part of the output voltage is'nsed for degenera tube. In Fig.3 differences in current, plate potential and hence the potential ofpoint 3 which is one terminal of the input circuit, and hence will reactthrough the input circuit to tend to return the grid to its originalgrid to cathode potential. In other wordsthe action will bedegenerative. Tube T operates in the same manner as tube T, andpotentiometer P is adjusted to a point such that with no input signalthe output terminals 1 and 8 will be at the same potential.- In thiscondition the amplifier is said to be balanced, and subsequentapplication of an input signal voltage will be indi-' cated or measuredin the output across the terminals I and 8.

If the gain of tube T were infinite, the control grid would be returnedto its initial potential. However, in practice the gain of tube T isfinite .hence the grid will not be restored to quite its originalpotential. This means that the change in the potential on the plate isnot quite as great as the input signal, hence the gain of this stage isnot quite unity. In the tube T there is no input signal applied to itsgrid, hence the drop in resistor I and across condenser 2 in the platecircuit of said tube will be identical to that in tube T if. the D. C.input voltage to tube T is zero. An indicating meter connected betweenthe output terminals I and 8 will read zero. A change in grid voltage oftube T changes its plate current only and hence results in a differencein the IR drops in the two plate load resistors and hence causes theoutput meter or indicator to read. In order to compensate for practicaldifferences in the plate resistances 'of tubes T and T there is providedthe potentiometer P. If the movable contact of P is so adjusted that theoutput meter will read zero when the input signal is zero, variations ofthe A. C. supply voltage will cause changes in the plate current, platevoltage and normal grid bias voltage of each tube to practically thesame extent so that these variations will not appear in the D. C.output.

In the circuit above described no voltage amplification is obtainable,the circuit being useful tube voltmeter where it is desirable to placelittle or no load on the measured circuit. The output of this circuit isessentially linear for D. C. input voltages and for A. C. input voltagesin phase with the power supply.

The circuit of Fig. 2 is practically the same as that'shown in Fig. 1except that variable portions of the voltage developed across therespective load resistors l and l aife applied to th respective controlgrids G aiirig'jG' i. e. only tion. The voltage amplification of thesingle stage shown in Fig. 2 is roughly proportional to the ratio of thewhole of the potentiometer to the upper part, within certain practicallimits set by the gain of the tube itself. The circuit is balanced byadjusting the slider 9 on the load resistance l for tube T.

Figs. 3 and 4 are variations of the preceding circuits in which tube- Tis self-biased as in Fig. 1 and the tube T'-is biased from the self-biasresistor of the tube T. The voltage gains obtainable in these circuitsapproach the mu of one the characteristics of tubes T and T arecompensated for by adjusting the plate load resistor N! of the latter,and the bias on tube T is obtained from the D. C. plate potential oftube T, and because of this,

.changes in the potential between terminal 3 and terminal 4 result insignal being applied to the grid of tube T. This applied voltage is suchthat the potential of the' plate of tube T varies a .and T is obtainedby means of the slider on potentiometer P and the entire signal isapplied to the grid of tube T and no change in plate current of tube Twill be occasioned by D. C. input signal.

Fig. 5 shows a modification of Fig. 2 in which the tubes are crossself-biased so that the circuit becomes regen'erative. Balance andregeneration are controlled by adjusting the sliders on plate resistorsI and I. This method of biasing is such that a change in the platepotential of tube T causes a change of grid potential of tube T which inturn causes a change in plate current and hence plate potential of tubeT which in turn reacts upon the grid of tube Tin the same direction asthe original grid change potential of tube T. It will therefore be seenthat this circuit is regenerative and that if the sliders on the plateresistors are set too near the plates the circuit will become unstable,that is, it will plop in one direction or the other. If, however, thesliders are set at a point just above this unstable point the gain ofthe circuit over a portion of this characteristic can theoretically bemade infinite. In Figure 5 adjustment of the slider on the plateresistors results in a change in balance of the circuit as well as achange in regeneration, and because of this it is extremely diificult tomake the appropriate adjustment. The circuit shown in Fig. 6 is designedto eliminate this difilculty.

In the circuit of Fig.6 in addition to the potentiometer P, I haveprovided a second potentiometer P2 connected between the anodes of tubesT and T. The filter R.C is connected between the mid-point of P2 and thecathode side of the line L2, and the grid of T is connected to a pointbetween R and C. A variable tap H on P2 is connected through resistor Rto the input terminal 3. If the circuit is properly balanced by theadjustment of the slider on potentiometer P, regeneration can beadjusted by means 'of slider II on potentiometer P2 without aflectingthe balance of the circuit, thus greatly simplifying operation of thecircuit. As the slider ll is adjusted to the lefttowards the plate oftube T the gain approaches from a large value to a value of slightlyless than twice the input while as slider H is moved to the right towardthe plate of tube T the gain approaches infinity tending to make thecircuit unstable. The regeneration or degeneration of this circuit maybe, controlled by adjusting slider ll without disturbing .the balance.Likewise the balance may be made by P without disturbing theregeneration. This circuit is easily adjustable and provides anexcellent null indicator for Wheatstone bridges, etc.

In the circuit shown in Figure 7 operation is identical to that ofFigure 4 with the exception that the grids are returned to an A. C.potential intermediate the A. C. potential of the two sides L'I and L2of the A. C. power supply. This introduces an A. C. bias into the gridcircuit so that the drop in the plate resistor may be. in-

creased to a value equal to the normal grid bias plus the peak value ofthe applied A. C. The

power transformer TR shown in the figure may be utilized for thispurpose, or else an autotransformer or a divider. The provision of theA. C. bias enables the current through the plate resistors to increaseand hence the normal D. C..

bias assumes a value in excess of th peak applied voltage-between pointsL2 and I2. This in turn enables the tubes T and T to operate at or nearthe center of their operating characteristic rather than relatively nearcut-off as is the case in the circuits shown in Figs. 1 to 6.

- It will of course be understood that this principle may be appliedequally as well to the foregoing circuits.

Fig. 8 shows a circuit in which the principles involved in the circuits.of Figs. 6 and 7 are combined, the former embodying regeneration, and

the latter the feature of operating the tube at or near the center ofits characteristic.

In Fig. 9 I have shown an improved A. C. operated self-biased balancedbridge circuit cascaded with a second identical balanced bridge circuit.Each of said bridge circuits is essentially identical regardingoperation and component parts to the circuit shown in Fig. 7. It will benoted that the A. C. and D. C. loads have been rearranged withoutchanging the fundamental operation of the bridge circuit so that oneside of the output can be connected to one side of the A. C. line. Ineach stage the plate of one tube (T in the first stage and T1 in thesecond) is held at ground D. C. potential while'the amplified T. C.output is measured by the assumed D. C. potential on the plate of theother tube (T in the first stage and T'z in the second) of therespective stages. Both grids of each stage are normally biased atground D. C, potential. Hence, the output of one bridge type amplifieris capable of being used to drive the grid of a following stageproviding a suitable A. C. filter such as R-C is used to transfer the D.C. from the plate of one stage to the grid of the next. The grids of thetubes have an A. C. bias applied to them to enable the potential dropsin the plate resistors to be great enough to allow the tubes to operateat or near the centers of their operating characteristic curves.

Electrically the only difierence between Figs. 7 and 9 is that the plateload of tube T has been placed on the other side of transformer TR. Thedrop across this load resistor P3 still supplies bias for both tubes Tand T. The D. C. load on tube T returns to the cathode in both cases. InFig. 7 the load resistance transformer winding is included in the returnpath while irf Fig. 9 the return is direct. The grid of tube T receivesits D. C. bias direct from the plate of tube T in both cases. In Fig, 7however an additional filter 5-6 is necessary to supply the correct A.C. bias. It will be noted that in both cases the elements of tubes T andT' receive identical 'A. C. and D. C. voltages.

The circuit shown in Fig, 10 shows how. the circuit of Fig. 9 can bemade regenerative. Balance adjustment is made by meansof P3independently of the regeneration, and the regeneration adjustment ismade independently of balance by adjusting P5.

Any of the above described circuits will measure A. C. of the samefrequency as, and in phase with the power supply voltage. Thus thesecircuits provide excellent balance or null indicators for Wheatstonebridges where the bridge current can be supplied from D. C. or from thesame A. C. source as the amplifier.

The circuits shown in Figs. '7, 8. 9 and 10 are normally operated on thestraight line portions of their characteristics, hence A. C. inputvoltages of frequencies other than the supply voltage produce novariations in the D. C. output voltages or currents. If the frequency ofeither the input or supply voltage is varied until a slow beat isobtained the output of the bridge will beat slowly, the maximum readingbeing a direct measure of the amplitude of the input voltage. The supplyvoltage amplitude will have practically no effect on the accuracy of thereading. If an oscillator is used to supply the plate or supply voltageto the bridge there is a slight tendency for the oscillator frequency tolook into step with that of the signal. This tendency is an advantage inthat as soon as the oscillator frequency is brought near that of thesignal the frequency of the oscillator will look into step with that ofthe signal. The proper phase relations between the signal and oscillatorfrequencies will automatically be held for reading. This greatlyfacilitates any measurements to be made and it has no apparent effect onthe accuracy of the readings. In other words if the above mentionedpower supply voltage is from a variable frequency oscillator this devicewill measure only D, C. or A. C. voltages of the same frequency as theoscillator. This means that the device can be certain changes orvariations will result in contact potential. To eliminate thisdifliculty, I have devised the circuit shown in Fig. 11 which isel'ectrically equivalent to and operates in much the same manner as oneof the balanced stages, T and T for example, of the amplifier circuitshown in Fig. 9. The circuit of Fig. 11 utilizes a pentagrid tube T3which may be of the type commonly known as 6L7, although it will beunderstood by those skilled in the art that a simple pentode such as the6J7 having in addition to the cathode K and the anode A the three gridsGfl,

G2 and G3 will be entirely suitable for the purpose. In the circuitutilizing the pentagrid tube T3 the grid G! acts as the grid of tube Tin Fig. 9 while the plate A acts as the plate of tube T of Fig. 9. Thegrid G3 of the pentagrid tube acts as the signal grid of tube T shown inFig. 9 and the grid G2 of the tube acts as. th plate of the tube T in-Fig, 9. The pentagrid tube shown is provided with the additional shieldgrid G4 connected to the grid G2 and with a suppressor grid G5 betweenthe shield grid G4 and the plate A, although as stated above the lattertwo grids are not essential to the operation of the circuit. It will benoted that in the circuit just described the cathode K serves to supplyspace current to both the grid G2 and the plate. A of the pentagrid tubeas shown or of the pentode if the grids G4 and G5 were omitted. Controlis obtained by changing the ratio of currents between the grid G2 andthe anode A. It will be understood that variations or changes in contactpotential at the surface of the cathode affect the total cathode currentbut do not necessarily afiect the ratio of currents between the grid G2and the plate A. Thus, since the output-is taken between said grld andplate, changes in the cathode current, when the circuitis balanced, donot cause any change in output. The circuit just described will bebalanced not only for line voltage variations but also for contactpotential variations.

While I have disclosed and described several without departing from thescope of my invention I as set forth in the appended claims.

What I claim is:

1. A system for the amplification of direct current voltages oralternating current voltages in phase with the power supply, comprisingmeans for emitting electrons, a pair of control grids and a pair ofelectron collecting electrodes, a power supply source of alternatingcurrent having a pair of line conductors, a conductive connection fromone of the line conductors to one of the collecting electrodes, acapacitive connection from the other line conductor to the electronemitting means, a load resistor connected between the second collectingelectrode and said other line conductor, means for adjustably connectingthe electron emitting means to said load resistor, an input circuitconnected to said pair of control grids, and an output circuit connectedto said collecting electrodes.

2. A system for the amplification of direct cur.- rent or alternatingcurrent voltages in phase with the power supply, comprising means foremitting electrons, a pair of control grids and a pair of electroncollecting electrodes, a power supply source of alternating currentincluding a transformer having a pair of line conductors connectedthereto, a conductive connection from one of the line conductors to oneof the collecting electrodes, a connection including a resistance fromthe other line conductor to the other collecting electrode, a connectionfrom the electron emitting means to an intermediate point on saidresistance, a source of input potentials connected to one of the controlgrids and to a mid-tap of the transformer secondary, and, an outputcircuit having terminals each connected to one of the electroncollecting electrodes.

- systems or modifications for carrying my inven- 3. In an alternatingcurrent operated A. C. or

D. C. voltage amplifier, a pair of electron discharge tubes each havingan anode, a cathode and a grid electrode, a source of alternatingcurrent including a transformer having a pair of line conductorsconnected thereto, a direct connection from one of said conductors tothe anode of one of said tubes,v a connection including a resistancefrom the other line conductor to the anode of the other tube, means foradjustably connecting the cathodes to said resistance, a

source of input potentials connected to one of the grid electrodes andto a mid-tap of the transformer secondary, and an output circuitconnected between the first mentioned line conductor and the anode ofsaid other tube.

4. The invention defined in claim 3 wherein a second pair of tubessimilarly connected as the first pair are coupled in cascade thereto bymeans of a resistance-capacity filter.

5. In an alternating current operated direct current amplifier, a pairof electron discharge tubes each having at least an anode, a cathode anda grid electrode, a source of alternating current including atransformer having a pair of line conductors connected thereto, a directconnection between one of the anodes and one of said line conductors, acapacitive connection from the other anode to the same line conductor, aconnection between each of the cathodes and the other line conductorthrough a resistance-condenser combination, a source of input potentialsconnected tothe grid electrode of one of said tubes and the mid-tap ofthe transformer secondary, a resistance connected between the anodes ofsaid tubes, a variable connection from said last mentioned resistance tothe grid electrode of the other tube, and an output circuit" havingterminals each connected to one of 'the anodes.

6. An A. C. operated D. C. amplifier system comprising an electrondischarge tube comprising a cathode, an anode and at least threeinterposed grid electrodes, an input source of D. C. potential adaptedto be connected to two of said grid electrodes, an output circuitconnected between the remaining grid electrode and the anode, an A. C.power supply source connected between said remaining grid electrode .andthe cathode,

and a load resistor connected between the cathode and the anode.

'7. An A. C. operated D. C. amplifier system comprising an electrondischarge tube comprising a cathode, an anode and at least fourinterposed grid electrodes, an input source of D. C. potential adaptedto be connected to the first and third grid electrodes, .an outputcircuit having a pair of terminals one connected 'to the second andfourth grid electrodes and the other connected to the anode, an A. C.power supply source connected between the latter grid electrodes and thecathode, and a load resistor connected be-,

tween the cathode and the anode.

' FRANCIS H. SHEPARD, JR.

